This invention relates to single-mode fiber optic couplers that are capable of effecting a relatively uniform coupling of light between fibers over a relatively broad band of wavelengths.
Fused fiber couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by inserting the fibers into a capillary tube prior to heating and stretching the fibers, thereby resulting in the formation of an "overclad coupler". To form an overclad coupler, the fibers are inserted into a tube, the tube is evacuated, and its midregion is heated and collapsed onto the fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling.
The cores become so small in the coupling region that their effect on propagation becomes very small. When the fiber cladding diameter becomes sufficiently small, the composite of the core and cladding functions as the light guiding portion of the waveguide in the coupling region, and the surrounding low index matrix material functions as the cladding. Power therefore transfers between the adjacent fiber claddings in the coupling region. The fundamental mode of that portion of the fiber that is in the coupling region (the core/clad/overclad waveguide) has a propagation constant different from the fundamental mode propagating in the fiber outside the coupling region. The term .beta..sub.CR will be used herein to refer to the propagation constant of the fundamental mode propagating in that portion of a coupler fiber that is in the coupling region. The propagation constant of the fundamental mode in the coupling region actually changes continuously with geometry. It is useful in order to obtain a qualitative understanding of the behavior of these couplers to consider a coupler having constant geometry in the coupling region with a lossless connection to input and output fibers.
Identical optical fibers were heretofore used to make a standard coupler, the coupling ratio of which is very wavelength dependent, i.e. if it exhibits 3 dB coupling at 1310 nm it cannot function as a 3 dB coupler at 1550 nm because of that wavelength dependence. A "standard coupler" might be characterized in terms of its power transfer characteristics in a window centered about 1310 nm, which is referred to as the first window. For example, a standard coupler might exhibit a coupling ratio that does not vary more than about .+-.5% within a 60 nm window.
An "achromatic coupler" is one wherein the coupling ratio is less sensitive to wavelength than it is for a standard coupler. There is no widely accepted definition of an "achromatic coupler". The least stringent definition would merely require an achromatic coupler to exhibit better power transfer characteristics than the standard coupler in the first window. More realistically, the specification is tightened by requiring an achromatic coupler to perform much better than the standard coupler in that first window, or to require it to exhibit low power transfer slopes in two windows of specified widths. These windows might be specified, for example, as being 100 nm wide and centered around about 1310 nm and 1530 nm. These windows need not have the same width; their widths could be 80 nm and 60 nm, for example. An optimally performing achromatic coupler would be capable of exhibiting low values of coupled power slope over essentially the entire single-mode operating region. For silica-based optical fibers this operating region might be specified as being between 1260 nm and 1580 nm, for example.
One type of achromatic coupler has been formed by employing fibers having different propagation constants for the fundamental mode in the coupling region, i.e. by using fibers of different diameter and/or fibers of different refractive index profile or by tapering or etching one of two identical fibers more than the other.
U.S. Pat. No. 5,011,251 teaches overclad achromatic fiber optic couplers wherein the coupled fibers are surrounded by matrix glass having a refractive index n.sub.3 that is lower than that of the fiber cladding material. The .beta..sub.CR of the two waveguides are different in the coupling region since the fibers have different cladding refractive indices. The difference between the refractive index n.sub.2 of the cladding of the first fiber and the refractive index n.sub.2 ' of the cladding of the second fiber is such that the coupler exhibits very little change in coupling ratio with wavelength over a relatively wide band of wavelengths. FIG. 1 shows the spectrum of a typical .DELTA..beta. achromatic coupler, made in accordance with U.S. Pat. No. 5,011,251. Whereas the insertion loss curves of the two outputs intersect near the centers of the two telecommunications windows, they diverge near the edges of those windows, and the separation between those curves at the edges of the windows is typically about 1 dB. This separation is referred to as "uniformity", and one key standards body, Bellcore, calls out in its document referred to as TA1209 a required uniformity of 1.0 dB and an objective of 0.5 dB.
U.S. Pat. No. 5,011,251 characterizes the tube refractive index n.sub.3 by the symbol .DELTA..sub.2-3, the value of which is obtained from the equation, .DELTA..sub.2-3 =(n.sub.2.sup.2 -n.sub.3.sup.2)/n.sub.2.sup.2'. The term .DELTA. is often expressed in percent, i.e. one hundred times .DELTA.. Commercially available single-mode optical fibers usually have a value of n.sub.2 that is equal to or near that of silica. If silica is employed as the base glass for the tube, a dopant such as B.sub.2 O.sub.3, and optionally fluorine, is added thereto for the purpose of decreasing the tube refractive index n.sub.3 to a value lower than n.sub.2. In addition to lowering the refractive index of the tube, B.sub.2 O.sub.3 also advantageously lowers the softening point temperature thereof to a value lower than that of the fibers. That patent teaches that when .DELTA..sub.2-3 is below about 0.2%, the amount of B.sub.2 O.sub.3 in a silica tube is insufficient to soften the tube glass in a 1.times.2 or a 2.times.2 coupler, whereby it excessively deforms the fibers during the collapse step. The value of .DELTA..sub.2-3 for standard couplers has therefore usually been between 0.26% and 0.35%, and to improve the reproducibility of the process of making achromatic overclad couplers of the type disclosed in that patent, .DELTA..sub.2-3 is preferably greater than 0.4%.
U.S. patent application Ser. No. 07/913,390 (D. L. Weidman-6) filed Jul. 15, 1992 teaches an overclad achromatic fiber optic coupler of the type wherein a plurality of single-mode optical fibers are fused together along a portion of their lengths to form a coupling region that is surrounded by a matrix glass body of refractive index n.sub.3. The coupler taper and n.sub.3 are such that the coupling constants of the coupler at two widely separated wavelengths are identical, thus giving achromatic performance. To achieve such achromatic performance, n.sub.3 must be lower than n.sub.2 by such an amount that the value of .DELTA..sub.2-3 is less than 0.125%, n.sub.2 being the fiber cladding refractive index. The value of .DELTA..sub.2-3 is preferably chosen so that nonadiabatic taper excess loss is kept below 0.5 dB. A discussion of nonadiabatic taper devices appears in the publication, W. J. Stewart et al., "Design Limitation on Tapers and Couplers in Single-Mode Fibers", Proc. IOPOC, 1985, pages 559-562. In order to meet this requirement, it appears that .DELTA..sub.2-3 must be lower than 0.125% and preferably lower than about 0.02%. As .DELTA..sub.2-3 becomes smaller, less refractive index-decreasing dopant is present in the silica-based matrix glass tube. The relatively hard matrix glass tube therefore deforms the fibers therein during the tube collapse step of the coupler forming process. Such fiber deformation may increase coupler excess loss, offsetting that decrease in the excess loss that is due to the decreasing of the taper steepness.